Melanin is dark brown pigments found in the eyes or on the skin, hair, etc. and plays positive roles in protecting the body or maintaining the body temperature, through blocking the penetration of ultraviolet radiation in a way that it absorbs the ultraviolet radiation over a certain amount. However, excessive exposure to ultraviolet radiation leads to oversecretion of melanin for blocking the skin penetration thereof, which often causes skin color changes. It is known that melanin is produced by the concurrence of melanin-producing enzymes (e.g., tyrosinase) and hormones.
Melanoma is a tumor formed by malignant alteration of melanin-producing cells (i.e., melanocytes). Although melanoma can occur in any body parts where melanocytes are present, the incidence thereof is the highest in the skin. And also, it is known that its malignancy is significantly high among various skin tumors. Although the incidence of melanoma is relatively low in the East in comparison with in the West, it is increasing year by year, showing the tendency that it begins to increase from twenty-year-old; and then rapidly increases in more than forty-year-old. It is thought that melanoma results mainly from genetic causes and/or exposure to ultraviolet radiation.
Meanwhile, Wnt is known as a cell signaling molecule secreted from various body cells, which is involved in three kinds of pathways, i.e., canonical Wnt pathway, non-canonical planar cell polarity pathway, and non-canonical Wnt/calcium pathway. Under normal conditions, Wnt is bound with Wnt antagonists such as Wnt inhibitory factor (WIF) and thus cannot be involved in the signaling. However, under certain circumstances in which WIF antagonists such as WIF cannot be expressed, Wnt is bound with the receptor, i.e., Frizzled, to generate the signaling. Secreted frizzled protein 5 (Sfrp5) is a secretory protein, which is known as one of the Wnt antagonists, similar to WIF. Sfrp5 contains the site having homology with the Wnt-binding site in Frizzled and thus controls the binding between Wnt and Frizzled proteins. The binding of Wnt with the receptor (Frizzled) in melanocytes leads to the β-catenin activation, thereby promoting the expression of MITF to control the melanin biosynthesis. Therefore, it is expected that the control of Wnt signaling pathways inhibits the expression of MITF, thereby being able to decrease pigmentation.